1. Field of the Invention
This invention relates to a wrap spring clutch and more particularly to an electromagnetically actuated wrap spring clutch that is capable of generating a high torque output using a low power input.
2. Disclosure of Related Art
A conventional, electromagnetically actuated, wrap spring clutch includes an output shaft, a shaft hub disposed about the shaft and connected for rotation therewith, and an input hub. The input hub is also disposed about the shaft, but may rotate independently of the shaft when the wrap spring clutch is deenergized. The clutch further includes a coil substantially disposed about the shaft hub and an annular wrap spring disposed about a portion of the input hub and a portion of the shaft hub. A first end of the wrap spring is connected to the input hub so that the spring rotates with the input hub. Energizing the coil establishes magnetic flux circuits or closed loops in the magnetically permeable portions of the clutch. Attractive forces arising from the flux draw a second end of the spring into contact with the shaft hub (which may be non-rotating at this point). Frictional forces restrain the second end of the spring from rotating. The difference in relative rotation between the first and second ends of the spring causes the spring to wrap down upon the shaft hub, thereby transmitting torque from the input hub to the shaft hub and output shaft.
Conventional clutches have long suffered from poor efficiency. The geometry of the magnetically permeable portions of a conventional clutch generally does not permit generation of a sufficient level of magnetic flux in response to relatively low power inputs. A decreased magnetic flux provides less attractive forces internal to the clutch (i.e., between the input and output components of the clutch) which, in turn, results in a diminished torque capacity. The inability to transmit a sufficient level of torque to the output shaft in response to low power inputs renders conventional clutches unsuitable for certain applications, such as automotive applications, where available power is limited as an initial matter, and in which large variations in voltage must be provided for (e.g., as a result of a reduction in the voltage output of a chemical battery during extremely cold weather).
There is thus a need for a wrap spring clutch that minimizes or eliminates one or more of the above-mentioned problems.